(1) Field of the Invention
The present invention relates to an optical fiber cable service system provided with video on demand service.
(2) Description of the Prior Art
Cable and telecommunication television system (hereinafter CATV) is a system for distributing high-quality television signals to each household's television set through a broad band transmission path such as a coaxial cable. Recently, with the progress of optical transmission technology, optical fiber cable service system has been applied to the arterial lines in order to expand the service area and to improve transmission performance. Furthermore, in order to realize B-ISDN, the establishment of optical network based on FTTH (Fiber To The Home) has been expected which provides each household with transmission and broadcasting service by means of a single optical fiber. Thus, the conventional Community Antenna Television System has been replaced by Cable and Telecommunication Television System.
Generally, analog SCM (Sub-Carrier Multiplexing) transmission system is employed for optical network where light is used as a main carrier. To be more specific, transmission signals such as video signals are modulated by means of a carrier having a pre-assigned frequency (sub carrier) in accordance with a predetermined modulation method (such as AM modulation or FM modulation), in order to form frequency-multiplexed signals in a predetermined transmission band (that is broadcasting band). The frequency-multiplexed signals are converted into optical intensity modulated signals and outputted. This allows multi-channel video signals to be transmitted concurrently through a single optical fiber transmission path, thereby realizing an optical cable service system.
The technique of optical analog video transmission systems are described in IEEE JOURNAL ON SELECTED AREA IN COMMUNICATIONS. VOL. 8. NO. 7. SEPTEMBER 1990 P.1249-1255 and 1991 NCTA Technical Papers P.13-19 and P.309-319.
The optical CATV system, on the other hand, offers video on demand (hereinafter VOD) service in addition to the general broadcasting service. The VOD service means to select a program desired by a subscriber from the program menu and sends the selected program to the subscriber.
FIG. 1 is a block diagram showing a general construction of an optical CATV system provided with the VOD service, and an optical network with FTTH is provided between the CATV center and each subscriber area. The analog SCM transmission system offers multi-channel video signals as broadcasting programs and video signals for VOD service to the subscribers from the CATV center.
The multi-channel video signal source 1001, which is provided with a plurality of analog base band video signal sources, modulates each video signal by means of a carrier having a pre-assigned frequency through a predetermined modulation method (for example AM modulation), frequency-multiplexes the modulated signal within a predetermined broadcasting band, and outputs it.
The video signal source 1101 for VOD service is provided with either an analog base band video signal source or a digital video signal source for VOD service having a plurality of programs.
The selector and modulator 1102 selects appropriate programs from the video signal source 1101 for VOD service in accordance with the VOD send-out request signal 1104 received from a subscriber, modulates the video signals of each of the selected programs by means of a carrier having a frequency pre-assigned within a predetermined VOD band, which does not overlap the broadcasting band. Then, the modulated video signals are frequency-multiplexed and outputted.
The frequency multiplexer 1003 frequency-multiplexes the output signal of the multi-channel video signal source 1001 and the output signal of the selector and modulator 1102, thereby outputting to the optical transmitter 1002.
The optical transmitter 1002, which is provided with a semi-conductor laser (LD) as a light source, amplitude-modulates current by means of an output signal of the frequency multiplexer, thereby outputting an optical intensity modulated signal.
The optical distribution units 1003, 1004a, 1004b, 1005a-1005d are located to form a tree-shape optical network, and lights outputted from the optical transmitter 1002 are distributed and transmitted to the eight optical receivers 1006a-1006h. The optical amplification units 1008, 1009a, and 1009b are optionally disposed on the optical transmission path when the number of subscribers is large, when the transmission distance is long, or when a large amount of received light power is necessary such as AM modulation method.
On the other hand, the optical receivers 1006a-1006h to be supplied in each subscriber's household re-converse the received optical signals into electric signals by means of direct detection type optical receivers, then output frequency-multiplexed video signals. The tuners 1007a-1007h select video signals requested by subscribers from among frequency-multiplexed signals to be outputted from the optical receivers 1006a-1006h, and demodulate the selected signals. When the VOD program menu includes programs requested by the subscribers, the tuners send out a send-out request signal 1104 for the programs to the selector and modulator 1102 in the CATV center. For the transmission of the send-out request signal 1104, conventional telephone network or the like is used.
In order to realize VOD service in the optical CATV system with analog SCM transmission system, it is necessary to assign a part of the broadcasting band to the band for transmitting VOD signals (hereinafter VOD band) or to provide an additional VOD band by expanding the entire transmission band. However, the VOD band width cannot be expanded without limitation in either way, so that the number of carriers for the VOD signals is restricted. This restriction lead to the limitation of the number of subscribers (which corresponds to the number of carriers) to be allowed to use the VOD service at the same time in a single optical CATV system. Consequently, it becomes impossible to raise the concurrent utility rate of the VOD service (the number of concurrent VOD service users/the number of the entire subscribers), which means that a great increase in the number of subscribers cannot be expected.
One method to solve such a problem is to divide all the subscribers into some groups and provide respective optical transmitting equipment or optical transmitting network for each group. This can reduce the number of subscribers per optical transmitting equipment, thereby raising the substantial concurrent utility rate of the VOD service.
FIG. 2 is a block diagram showing a construction of the optical CATV system provided with the VOD service in which subscribers are divided into some groups in order to improve the concurrent utility rate, and in which the construction shown in FIG. 1 is provided for each group.
As shown in FIG. 2, the entire subscribers are divided into a group A and a group B, and the households of the subscribers in the group A are provided with the optical receivers 1006a-1006d, the tuners 1007a-1007d, and the households of the subscribers in the group B are provided with the optical receivers 1006e-1006h, and the tuners 1007e-1007h.
The groups A and B are respectively provided with the multi-channel video signal sources 1001a and 100lb, the video signal sources 1101a and 1101b for VOD service, the selectors and modulators 1102a and 1102b, the frequency multiplexing units 1103a and 1103b, and the optical transmitters 1002a and 1002b as the optical transmitting equipment. The optical transmission paths in the groups A and B are respectively provided with the optical amplification units 1009a and 1009b, the optical distribution units 1004a, 1005a, 1005b, and 1004b, 1005c, 1005d.
However, it is extremely costly to enlarge the optical transmitting equipment or to relay the optical transmitting network. Hence, it is a current issue to achieve VOD service with a high concurrent utility rate at a low cost in the present CATV system having increasing subscribers.